1. Field of the Invention
The present invention relates to a process for producing low carbon content cold rolled steel strips having excellent deep drawing properties by a short-time continuous annealing. By low carbon content steel is meant steel having a carbon content of from 0.003 to 0.08%.
Cold rolled steel strip is widely used for cold forming articles such as press-formed automobile parts, and as such the strip is required to have an excellent press-forming property.
In order to improve the general workability of steel strip, it is necessary to allow full growth of grains in the steel and, on the other hand, to minimize the amount of dissolved or solute carbon in the steel. Further, with respect to the deep-drawability of the steel, it is desirable that the average plastic-strain ratio r be large. The r value is related to the crystal orientation and the larger the value of the {111} component the larger the r value.
Cold rolled steel strip is generally produced by a process which comprises the essential steps of hot rolling and cold rolling the steel to form the strip and annealing the thus rolled strip. For satisfactorily increasing the grain size and the r value, it is effective to slowly heat the steel strip and hold it for a long period of time at the annealing temperature, and for reducing the amount of solute carbon, it is effective to subject the steel strip after annealing to slow cooling so as to precipitate substantially all of the carbon content at the grain boundaries.
2. Description of Prior Art
Conventionally, batch annealing has been widely used for production of cold rolled steel strip because the above described conventional annealing conditions can be easily achieved by the use of a batch type annealing furnace. Although batch annealing has been considered to be most suited for obtaining excellent workability of steel strip, it has a critical disadvantage that it takes a long period of time to complete the treatment, and hence considerably lowers the production efficiency.
Therefore, considerable effort has been made to develop new processes such as continuous annealing, for producing cold rolled steel strip having excellent workability by a short-time treatment, and in recent years some continuous annealing processes have been disclosed, for example, in Japanese Patent Publication Sho 42-11911, Japanese Laid-open Patent Applications Sho 50-72816, Sho 50-125918 and Sho 51-32418.
However, it has never been understood that these prior art processes have certain defects as discussed below.
In the conventional continuous annealing process, the main consideration has been to achieve a satisfactorily large grain size, and it has been considered that a longer holding time produces better grain size characteristics. This is in spite of the fact that continuous annealing was developed for the purpose of shortening the treating time. This is clearly illustrated in the above-mentioned prior art publications in which only the lower limits of the annealing time are defined.
Indeed, a longer annealing time promotes full growth of grains so that a large grain size can be obtained, and this does have certain advantages. However, if the annealing time is excessively long, the carbides which have been precipitated in the hot rolled steel strip will be dissolved during the annealing process, thereby increasing the amount of solute carbon, and thus causing deterioration of the workability of the cold rolled steel strip. To avoid this drawback, in the conventional continuous annealing process, the cold rolled steel strip, after annealing, is subjected to an over-ageing treatment at about 400.degree. C. for a considerably long period of time so as to precipitate the solute carbon again as carbides.
Thus, the conventional continuous annealing processes include the following contradictory considerations:
(a) a longer annealing time produces a larger grain size, thus improving the workability of cold rolled steel;
(b) a shorter annealing time is more effective in preventing dissolving of carbides formed in the hot rolled steel strip, thus shortening the subsequent over-ageing treatment.
However, only the factor (a) has been taken into consideration in the conventional processes, and no consideration has been given to the factor (b).